Method of reducing corrosion potential and stress corrosion cracking susceptibility in nickel-base alloys

ABSTRACT

A method for reducing in situ the electrochemical corrosion potential and susceptibility to stress corrosion cracking of a nickel-base alloy and boiling water nuclear reactor components formed therefrom when in contact with high temperature water. The method comprises the steps of: adding a metal hydride to the high temperature water; dissociating the metal hydride in the high temperature water to form a metal and at least one hydrogen ion; and reducing the concentration of the oxidizing species by reacting the hydrogen ions with an oxidizing species, thereby reducing in situ the electrochemical corrosion potential of the nickel-base alloy. The method may further include the steps of reacting the metal with oxygen present in the high temperature water to form an insoluble oxide and incorporating the metal into the surface of the nickel-base alloy, thereby reducing the electrical conductivity of the surface of the nickel-base alloy. A nickel-base alloy component having a reduced electrochemical corrosion potential is also disclosed.

This application is a division of application Ser. No. 09/681,148 filedJan. 29, 2001, now U.S. Pat. No. 6,488,782.

BACKGROUND OF THE INVENTION

The present invention relates to protecting nickel-base alloys andcomponents thereof from stress corrosion cracking when in contact withhigh temperature water. More particularly, the invention relates toprotecting nickel-base alloy components of a boiling water reactor (BWR)from stress corrosion cracking when in contact with high temperaturewater. Even more particularly, the invention relates to protectingnickel-base alloy components of a boiling water reactor (BWR) fromstress corrosion cracking when in contact with high temperature water bylowering the electrochemical corrosion potential of the nickel-basealloy components.

Nickel-base alloys, such as alloys 600, 690, 182, 82, X750, 718, andsuperalloys, have found applications in both boiling water nuclearreactors (hereinafter referred to as BWRs) and pressurized water nuclearreactors (hereinafter referred to as PWRs). These applications includeuse in many structural components found in nuclear reactors, such as,but not limited to, pipes, bolts, and weld material. Water for coolingthe reactor core and extracting heat energy therefrom circulates withinthe BWR reactor pressure vessel, with about 15% of the water charged tosteam. Inside the BWR reactor pressure vessel, the steam and circulatingwater typically have an operating pressure and temperature of about 7MPa and 288° C., respectively. For a PWR, the circulating water has anoperating pressure of about 15 MPa and a temperature of about 320° C. Inthe presence of water and/or steam under such high pressures andtemperatures, components formed from nickel-base alloys are subject tointergranular stress corrosion cracking (hereinafter referred to asIGSCC), more commonly, or generically, referred to stress corrosioncracking (hereinafter referred to as SCC).

Stress corrosion cracking (SCC) of nuclear reactor components has longbeen a concern. As used herein, SCC refers to cracking propagated by theapplication of static or dynamic tensile stresses in combination withcorrosion at a crack tip. The stresses encountered within BWR and PWRpressure vessels include those arising from the operating pressure forcontainment of the high temperature water in a liquid state, vibration,differences in thermal expansion, residual stress from welding, andfabrication-related sources of stress. Various materials andenvironmental conditions, such as water chemistry, welding, surfacenature, crevice geometry, heat treatment, radiation, and other factorscan also increase the susceptibility of reactor components to SCC.

Boiling water reactors use water as a means of cooling nuclear reactorcores and extracting heat energy produced by such reactor cores. Stresscorrosion cracking is of particular concern in BWRs, as radiolyticdecomposition of the high temperature water in the BWR core increasesthe concentrations of oxidizing agents, such as O₂ and H₂O₂, in the hightemperature water that circulates through the reactor. Consequently, thelikelihood of extensive SCC in materials that are exposed to the hightemperature reactor water is substantially increased. SCC can eventuallylead to the failure of a nickel-base alloy structural component, such asa bolt. The premature failure of such components may lead to repeated orearly shutdown of the reactor for part replacement or repair, thusreducing the amount of time the reactor is available for powergeneration.

The electrochemical corrosion potential (hereinafter referred to as ECP)affects the susceptibility of BWR components to SCC. The ECP is themixed potential associated with the equilibrium of redox reactionsoccurring on a metal surface and the metal dissolution, and is dependentupon the amounts of oxidizing and reducing species present in thereactor water. In BWR reactor water, cathodic currents associated withthe reduction of oxygen and hydrogen peroxide are balanced by anodiccurrents involving hydrogen oxidation and corrosion of metalliccomponents.

Several approaches have been adopted to reduce SCC by lowering the ECPof the reactor water. In one such method, commonly referred to ashydrogen water chemistry (HWC), gaseous hydrogen is added to the BWRfeedwater. Hydrogen addition reduces the oxidant concentrations, andthus reduces SCC susceptibility, by recombining with dissolved oxidantsthat are produced by the radiolysis of water in the reactor core. Onedisadvantage of HWC is that large amounts of hydrogen are needed tosufficiently lower the concentration of dissolved oxygen and to achievea low corrosion potential. In addition, HWC can also increase radiationlevels in the reactor steam by increasing the volatility of radioactiveN¹⁶.

A second approach, known as noble metal technology (NMT), reduces thesusceptibility of BWR components to stress corrosion cracking bylowering the corrosion potential more efficiently; i.e., by reducing theamount of hydrogen required to lower the electrochemical corrosionpotential. The objective of NMT is to improve the catalytic propertiesfor hydrogen/oxygen recombination on metal surfaces. Niederach (U.S.Pat. No. 5,130,080), Andresen and Niederach (U.S. Pat. Nos. 5,135,709and 5,147,602), and Hettiarachchi (U.S. Pat. No. 5,818,893) havedisclosed various NMT application methods, such as the thermal sprayingof noble metal and noble metal alloy coatings on reactor components andnoble metal chemical addition on metal reactor components. The NMTprocess lowers the corrosion potential to below −500mV_(SHE) (standardhydrogen electrode) with a small amount of hydrogen addition. Whencombined with hydrogen addition in stoichiometric proportions orgreater, complete recombination of oxygen and hydrogen peroxide on thecatalytic surface of the noble metal is achieved and the corrosionpotential is dramatically reduced.

Other methods, which do not require the addition of hydrogen to reducethe corrosion potential of reactor components—particularly of steelvessels and piping—have been developed. Because electrically insulatingfilms on metal surfaces reduce the corrosion potential, the ECP is alsoaffected by the electrical conductivity of oxide films formed on metalsin high temperature water. By lowering the electrochemical corrosionpotential of metal components, the susceptibility of such materials toSCC can be significantly reduced. Andresen and Kim (U.S. Pat. No.5,465,281) and Hettiarachchi (U.S. Pat. No. 5,774,516) teach a method ofreducing the electrochemical corrosion potential of steel exposed tohigh temperature water with an insoluble and electrically non-conductivematerial, such as zirconia (ZrO₂), alumina (Al₂O₃), or yttria-stabilizedzirconia (YSZ) powders. However, air plasma spray coatings generallymust be applied to the components either prior to installation or duringa power outage. Moreover, it is difficult to achieve complete coveragewith injection of insoluble chemical compounds into the reactor water.

More recently, Andresen and Kim (U.S. Pat. No. 6,024,805) have disclosedan in situ method of reducing the ECP and thus lowering thesusceptibility of stainless steel that is exposed to high temperaturewater to stress corrosion cracking. The method includes the addition ofa metal hydride to the high temperature water.

The prior art has focused on reducing in situ the corrosion potential ofstainless steel pressure vessels and piping within BWRs. Whileinsulating oxide coatings have been applied to nickel-base alloys, todate no attempt has been made to reduce in situ the susceptibility ofnickel-base alloys to stress corrosion cracking by lowering theelectrochemical potential of the alloy in the BWR without addinghydrogen to the reactor water. Therefore, what is needed is a method oflowering the susceptibility of nickel-base alloys that are exposed tohigh temperature water to SCC. What is also needed is a method oflowering the ECP of nickel-base alloys exposed to high temperaturewater, thereby mitigating stress corrosion cracking in such alloys.Finally, what is also needed is a nickel-based alloy having a reducedcorrosion potential, and thus, a reduced susceptibility to stresscorrosion cracking.

BRIEF SUMMARY OF THE INVENTION

The present invention meets these needs and others by providing a methodof reducing in situ the ECP of a nickel-base alloy that is in contactwith high temperature water, such as in, but not limited to, thepressure vessel of a BWR, without adding hydrogen to the water. Thepresent invention also provides an article formed from a nickel-basealloy having a reduced corrosion potential.

Accordingly, one aspect of the present invention is to provide a methodfor reducing in situ an electrochemical corrosion potential of anickel-base alloy having a surface that is in contact with hightemperature water, the electrochemical corrosion potential beingproportional to the concentration of oxidizing species present in thehigh temperature water. The method comprises the steps of: adding ametal hydride to the high temperature water, the metal hydride beingcapable of dissociating in water; dissociating the metal hydride in thehigh temperature water to provide a metal and hydrogen ions; andreducing the concentration of the oxidizing species by reacting thehydrogen ions with the oxidizing species, thereby reducing in situ theelectrochemical corrosion potential of the nickel-base alloy.

A second aspect of the present invention is to provide a method ofreducing the in situ susceptibility of a nickel-base alloy componentthat is in contact with high temperature water in a boiling waternuclear reactor to stress corrosion cracking. The method comprises thesteps of: adding a metal hydride to the high temperature water, themetal hydride being capable of dissociating in water; dissociating themetal hydride in the high temperature water to provide a metal andhydrogen ions; and incorporating the metal into an oxide layer disposedon a surface of the nickel-base alloy, the oxide layer being in contactwith the high temperature water, wherein the electrical conductivity ofthe surface of the nickel-base alloy is reduced, and wherein theresulting decrease in the electrical conductivity reduces in situ thesusceptibility of stress corrosion cracking of the nickel-base alloycomponent.

A third aspect of the present invention is to provide a method ofreducing in situ susceptibility of stress corrosion cracking of anickel-base alloy component that is in contact with high temperaturewater in a boiling water nuclear reactor, the susceptibility of stresscorrosion cracking being proportional to the concentration of oxidizingspecies present in the high temperature water. The method comprises thesteps of: adding a metal hydride to the high temperature water, themetal hydride being capable of dissociating in water; dissociating themetal hydride in the high temperature water to provide a metal andhydrogen ions; reducing the concentration of the oxidizing species, theoxidizing species being selected from the group consisting of O₂ andH₂O₂, by reacting the hydrogen ions with the oxidizing species, therebyreducing in situ the electrochemical corrosion potential of thenickel-base alloy; and incorporating the metal into an oxide layerdisposed on a surface of the nickel-base alloy, the oxide layer being incontact with the high temperature water, wherein the electricalconductivity of the surface of the nickel-base alloy is reduced, andwherein the resulting decrease in the electrical conductivity reduces insitu the susceptibility of stress corrosion cracking of the nickel-basealloy component.

Finally, a fourth aspect of the present invention is to provide anickel-base alloy component having a reduced susceptibility to stresscorrosion cracking when said nickel-base alloy component is in contactwith high temperature water. The nickel-base alloy component comprises:a nickel-base alloy; a surface and a layer disposed thereon, the layerbeing formed from an oxide of a first metal and being in contact withthe high temperature water; and at least a second metal incorporated inthe layer, wherein the second metal is incorporated in situ into thelayer by adding a hydride of the second metal to the high temperaturewater, dissociating the hydride in the high temperature water, andincorporating the second metal into said oxide of said first metal.

These and other aspects, advantages, and salient features of theinvention will become apparent from the following detailed description,the accompanying figures, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the electrochemical corrosion potential (ECP) of 304stainless steel and nickel-base alloy 690 as a function of oxygenconcentration in 288° C. water with the addition of zirconium hydrideand without the addition of zirconium hydride; and

FIG. 2 is a plot of the effect of the addition of zirconium hydride(ZrH₂) on hydrogen and oxygen concentrations in 288° C. water.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that terms such as “top,” “bottom,”“outward,” “inward,” and the like are words of convenience and are notto be construed as limiting terms.

Referring to the figures and examples in general, it will be understoodthat the illustrations are for the purpose of describing a preferredembodiment of the invention and are not intended to limit the inventionthereto.

The present invention discloses a new approach for achieving lowcorrosion potentials on nickel base alloys, such as alloys 600, 690,182, 82, 718, X750, weld metals, nickel-base superalloys, and the like,that are widely used in BWRs. Unlike other methods found in the priorart for reducing the electrochemical corrosion potential (ECP) ofmaterials used in BWRs, the method of the present invention does notrequire that hydrogen be separately added. In the present invention, ametal hydride MH_(n) is injected into the reactor feedwater. The metalhydride can be directly injected as either a powder or slurry, or bysuspending a metal hydride powder into the feedwater.

Once introduced into the reactor feedwater, the metal hydride MH_(n)dissociates in high temperature water to yield the elemental metal andat least one hydrogen ion H⁺, as represented by the general reaction

MH_(n)→M+nH⁺+ne⁻(1).

The high radiation flux within the BWR pressure vessel enhances the rateof dissociation of the metal hydride compound. In the present invention,the metal hydride that is used to lower the ECP of the nickel-base alloyis a hydride of a metal selected from the group consisting of hafnium,lanthanum, lithium, manganese, molybdenum, sodium, niobium, neodymium,palladium, praseodymium, plutonium, samarium, strontium, tantalum,thorium, titanium, uranium, vanadium, yttrium, and zirconium.

The concentrations of oxidizing species, such as O₂ or H₂O₂, in the hightemperature water are reduced as the oxidizing species react with thehydrogen ions released by the dissociation of the metal hydride MH_(n),thus lowering the ECP of Ni-based alloy components in the BWR. Oxygen,for example, reacts with the hydrogen ions released by the hydridedecomposition, to yield water according to the reaction

4e⁻+4H⁺O₂→2H₂O (2).

By reducing the concentrations of such oxidizing species, the corrosionpotential and susceptibility of the nickel-base alloys that are exposedto the high temperature reactor water to stress corrosion cracking iscorrespondingly lowered.

In addition to reducing the ECP by providing hydrogen ions that reactwith oxidizing species present in the high temperature water, thepresent invention further reduces the ECP by incorporating the metalreleased by the decomposition of the metal hydride into the thin oxidelayer that is present on the surface of the nickel-base alloy.

The neutral metal atom that is produced by the metal hydridedissociation of equation 1 may be readily ionized according to thereaction

M→M^(n+)+ne⁻ (3).

The metal ion M^(n+) may then react with O²⁻ ions in the hightemperature water to form an oxide:

Mn^(n+)O²⁻→MO(s) (4).

which is then deposited on the thin oxide layer that is present on thesurface of the nickel-base alloy component. Alternatively, the metal ionM^(n+) may be incorporated into the thin oxide layer by reacting with asubstance—typically, oxygen or an oxide—located in the oxide layer. Theincorporation of the metal into the oxide layer decreases the electronicconductivity of the oxide film and eventually decreases the ECP of thenickel-base alloy components, thus decreasing the susceptibility ofthese components to stress corrosion cracking. Incorporation of themetal into the thin oxide layer on the nickel-base alloy surfacetypically occurs when hydrides of the metals found in groups IIIB, IVB,and IVB of the periodic table are injected into the reactor feedwater.Preferably, zirconium hydride (ZrH₂), titanium hydride (TiH₂), scandiumhydride, hafnium hydride, niobium hydride, and vanadium hydride (VH₂)are the metal hydrides used for the incorporation the metal into thethin oxide layer.

The present invention offers the advantage providing hydrogen ions toreduce the concentration of oxidizing species within the reactor waterand reduce the corrosion potential of the nickel-base alloy reactorcomponents while either reducing or eliminating the need to add gaseoushydrogen. Metal hydride injection results in a more even distribution ofhydrogen than that obtained when hydrogen gas is added, therebyproviding a greater overall reduction of oxidizing agents, such as O₂ orH₂O₂, in the water.

Moreover, the present invention further reduces the corrosion potentialand susceptibility of nickel-base alloy reactor components to SCC byuniformly incorporating metals in situ into the oxide layer that ispresent on the surface of the nickel-base alloy. Air plasma spraying ofnoble metals and oxide coatings are generally unable to be applied insitu during plant operation.

The following example serves to illustrate the features and advantagesof the present invention.

EXAMPLE 1

Electrochemical corrosion potential (ECP) measurements were performed ontest electrodes of nickel-base alloy 690, 304 stainless steel, andzircaloy-2. The test electrodes were first pre-oxidized for 2 weeks in288° C. water containing 200 ppb oxygen prior to the ECP measurement.The ECP of each test electrode was then measured for 2 days in 288° C.water containing 300 ppb oxygen. Suspensions of ZrH₂ were then injectedinto the recirculating water loop, and argon gas was purged through thisinjection solution during experiments. Oxygen and hydrogenconcentrations in the outlet water were measured simultaneously with theECP measurement.

FIG. 1 is plot of the electrochemical corrosion potential (ECP) of 304stainless steel and nickel-base alloy 690 as a function of oxygenconcentration in 288° C. water. As can be seen from FIG. 1, the additionof ZrH₂ decreased the ECP of the nickel-base alloy 690. No change inECP, however, was observed on the zircaloy 2 specimen, which had alreadyformed the insulating oxide (ZrO₂) on the surface. FIG. 2 shows theeffect of ZrH₂ addition on the oxygen and hydrogen concentrations in theoutlet water measured simultaneously with the ECP measurement. The ECPfor 304 SS was measured as a reference.

As seen in FIG. 1, the ECP of nickel-base alloy 690 decreases to about−200 mV_(SHE) with the addition of ZrH₂. The concentration of hydrogenproduced by the decomposition of ZrH₂ in the high temperature waterincreases with increasing ZrH₂ injection time, as seen in FIG. 2. Theresults shown in FIGS. 1 and 2 show that ZrH₂ addition provides variousbeneficial effects on nickel-base alloys that are in contact with hightemperature water. First, the hydrogen ions provided to the hightemperature water by decomposing the metal hydride reduce the oxidantconcentrations. Second, the metal from the metal hydride reacts in thehigh temperature water to form insoluble metal oxides which are thenincorporated into the thin oxide layers that are present on the surfaceof nickel-base alloy components, thereby decreasing the electricconductivity of the component. These processes decrease the ECP ofnickel-base alloys that are exposed to the high temperature water,thereby reducing the susceptibility of the alloys to SCC in hightemperature water.

While various embodiments are described herein, it will be appreciatedfrom the specification that various combinations of elements, variationsor improvements therein may be made by those, skilled in the art, andare within the scope of the invention. For example, the methods of thepresent invention are applicable to a wide range of water chemistryenvironments where a low corrosion potential leads to reduced SCCsusceptibility.

What is claimed is:
 1. A nickel-base alloy component having a reduced susceptibility to stress corrosion cracking when said nickel-base alloy component is in contact with high temperature water, the nickel-base alloy component comprising: a) a nickel-base alloy; b) a surface and a layer disposed thereon, said layer being formed from an oxide of a first metal and being in contact with said high temperature water; and c) at least a second metal incorporated in said layer, wherein said second metal is incorporated in situ into said layer by adding a hydride of said second metal to the high temperature water, dissociating said hydride in the high temperature water; and incorporating said second metal into said oxide of said first metal.
 2. The nickel-base alloy component of claim 1 wherein said hydride is a hydride of a metal selected from the group consisting of Group IIIB metals, Group IVB metals, and Group VB metals.
 3. The nickel-base alloy component of claim 2, wherein said hydride is a hydride selected from the group consisting of zirconium hydride, titanium hydride, vanadium hydride, and mixtures thereof.
 4. The nickel-base alloy component of claim 1, wherein said nickel-base alloy is selected from the group consisting of alloy 600, alloy 690, alloy 182, alloy 718, alloy X750, weld metals, and nickel-base superalloys.
 5. The nickel-base alloy component of claim 1, wherein said nickel base alloy component is a boiling water nuclear reactor component. 